For an ideal utility power delivery system characterized by an AC line voltage V.sub.in, a line impedance Z.sub.in, and an AC current I.sub.in, the power delivered to a load is the dot product of the voltage across the load and the current running through the load, or P.sub.out =V.sub.out.multidot.I.sub.out cos(q) where q represents the phase difference between the voltage across the load and the current running through the load. Maximum power is thus most efficiently delivered to the load when the phase angle of the current coincides with the phase angle of the voltage at the load (cos(q)=1), corresponding to a power factor of one.
Power distribution systems typically supply power for loads which are both purely resistive as well as loads which exhibit impedances having substantial reactive components, for example electric motors, power supplies (converters), fluorescent and HID lighting, and the like. The reactive component of load impedance, whether capacitive or inductive, shifts the phase angle of the current running through the load with respect to the supply voltage, resulting in a proportional decrease in power factor at the load and corresponding reduction in the efficiency of the power distribution system. Stated another way, for power factors less than one, an electric utility power company must provide more "power" than is actually consumed by the various loads connected to the power distribution system.
Power factor correction techniques are generally well known. Typically, electronic devices having a reactive component equal in magnitude but opposite in sign to the reactive component of the impedance exhibited by the load are placed in parallel with the load; for example, the common technique is to place a bank of capacitors in parallel across an inductive motor to cancel the inductive reactance produced by the motor. In this way, the power factor is corrected to unity, and the overall impedance of the load appears purely resistive from the perspective of the source (e.g. the power company).
Reactive loads which draw current in a nonlinear fashion are considerably more problematic, however, particularly to the extent that the line current harmonics resulting from the nonlinearities are reflected back to the power source.
Presently known power converters, for example AC-DC converters, and in particular those employing silicon rectifiers, tend to exhibit discontinuous supply line current drawing characteristics, i.e., nonlinear load characteristics. In such systems, current flowing through the load is typically zero until the AC supply voltage exceeds a first DC conduction threshold voltage defined by, inter alia, the rectifier circuit. Thereafter, the current through the load increases sharply, limited primarily by line impedance. The current level again returns to zero as the AC supply voltage drops below a second DC conduction threshold voltage, typically defined by the filter capacitor and the rectifier circuit. Consequently, the diode conduction angle is restricted to a relatively small angular region centered around .pi./2 radians in the AC line voltage, which constitutes a comparatively small fraction of the total potential conduction angle provided by a rectified sine wave. As a result of this reduced conduction angle, substantially all of the power consumed by the load is drawn during a small portion of the AC cycle, resulting in very high current peaks and, hence, very high peak-to-RMS current ratios (crest factors). In addition, these nonlinear current drawing characteristics produce high frequency harmonics which are reflected back to transformer cores in the distribution system.
These high crest factors and harmonic components negatively affect the utility company's ability to adequately provide power to an increasingly complex universe of consumers. For example, it is known that transformer core losses are a function of the square of the frequency of reflected current harmonics. Moreover, high crest values require that the total generating capacity and the transformers used by utilities to produce electrical power be of sufficient size to supply the needed crest current. The capital cost to the utility companies to provide the extra generating capacity and large transformers is extraordinary.
Moreover, as more nonlinear electronic equipment (e.g., computers) is connected to existing power distribution systems, high frequency noise generated by nonlinear loads tends to interact with other electronic equipment on the same power distribution line, which may result in a degradation in the reliability of these devices, for example manifesting as a loss of data system integrity for computer systems.
Passive methods to reduce the nonlinear current drawing effects of power consumption devices typically involve the use of inductive type filters used in conjunction with the load. These filters, however, are quite large and expensive.
Active methods to reduce nonlinear current consumption have also been constructed, but they too are unsatisfactory in several regards. For example, many active correction schemes employ a feedback loop using a non-filtered voltage wave shape to correct the current waveform which is detected at the AC input. While these active systems generally yield good power factor characteristics, they tend to amplify the deleterious effects of harmonic distortions present on the voltage waveform of the AC input line.
An apparatus for suppressing or eliminating nonlinear current drawing characteristics of a load is thus needed which overcomes the shortcomings of the prior art.